The use of fluxes is well known in the field of metallurgy and these fluxes fulfill various functions.
Fluxes can be used to form a protecting layer at the surface of an alloy to prevent oxidation. When fluxes contain chemical active agents, they can be used to clean furnace walls by softening accumulated layers of corundum. Some exothermic fluxes are also used for cleaning dross and removing aluminum trapped in oxide layers.
Fluxes that are based on alkali chlorides and alkaline-earth chlorides are also used for the refining of alloys. Those skilled in the art generally define refining as the removal of alkali and alkaline-earth metals, non metallic inclusions and hydrogen from the alloys.
Sodium and calcium are always present as impurities in aluminum obtained from the Hall-Héroult process. Lithium fluoride is often added to the electrolytic bath to improve the efficiency of cells. However, a small amount in the metallic state is found dissolved in the aluminum. These impurities entail quality issues. For example, in an alloy containing magnesium, the presence of sodium may interfere during the hot rolling processes. The presence of sodium in aluminum and silicon alloys neutralize the effect of phosphorus used for the refining of grains. For the above-mentioned reasons, the use of fluxes containing sodium is not recommended for aluminum and its alloys, more particularly for aluminum alloys comprising a magnesium content higher than 3% by weight or a silicon content higher than 10% by weight.
Also, the presence of hydrogen in too high concentration may lead to a too high porosity of the aluminum during its solidification. During the recycling of aluminum, the presence of non metallic inclusions is important.
Reactional kinetics for the withdrawal of calcium and sodium in an aluminum alloy have been well studied. Naturally, in these alloys, both impurities disappear according to a kinetic of order 1 for small concentrations and order 0 for high concentrations. Because of its high vapor tension, sodium oxidizes itself more rapidly than calcium, that is why calcium is used during cleaning tests. The addition of fluxes involves an increase of reactional constants and thereby a faster reduction of the content in impurities. Mixing also has a non negligible effect on the reduction of impurities. Mixing accelerates the withdrawal of impurities by increasing the contact between impurities and the salt flux.
MgCl2 is one of the chemical active agents used for the withdrawal of impurities in alloys. Its concentration has a direct effect on the kinetic of withdrawal of calcium and sodium. Its melting point is 714° C., but in common fluxes, it is mixed with other salts to obtain a melting point between 400 and 550° C. However, MgCl2 is hygroscopic and can not be exposed for a long period of time to the surrounding air. Fluxes obtained by fusion of salts comprising magnesium chloride have hygroscopic properties. Consequently, the packaging is an important factor in limiting the absorption of humidity during the manufacturing of such fluxes.
There are examples of fluxes that are based on magnesium chloride. U.S. Pat. No. 1,377,374 relates to the use of a flux having an equimolar composition of sodium chloride and magnesium chloride for the production of manganese or magnesium alloys. U.S. Pat. No. 1,754,788 relates to the use of this same flux in a process for the cleaning of magnesium. U.S. Pat. No. 1,519,128 relates to the addition of calcium chloride to this composition and U.S. Pat. No. 2,262,105 relates to the addition of potassium chloride and magnesium oxide in addition to the calcium chloride. U.S. Pat. No. 5,405,427 mentions a flux based on sodium chloride, magnesium chloride, potassium chloride and carbon for the treatment of metal.
The article entitled “Salt Fluxes for Alkali and Alkali and Alkaline Earth Element Removal from Molten Aluminum” by David H. DeYoung shows the use of a ternary salt based on magnesium chloride, sodium chloride and potassium chloride for the removal of sodium, calcium and lithium from aluminum alloys. However, the article entitled “The Treatment of Liquid Aluminum-Silicon Alloys” by Gruzleski et al., pp. 204-205 indicates that it is important to those skilled in the art, not to use fluxes containing sodium salts. Therefore, even if a ternary flux salts having low content in sodium salts may be tolerated, those skilled in the art are expressly invited to avoid using sodium salts.
Initially, the refining of aluminum was carried out by bubbling of chlorine and argon in the liquid metal. However, this created environmental problems due to emissions of chlorine, chlorhydric acid and particles in suspension. The use of salt fluxes was later adopted as a more ecologically-friendly solution.
The refining fluxes are usually composed of alkali chlorides or alkaline-earth chlorides, which are mixed to obtain melting points that are lower than the operating temperature of alloys—the melting point of pure compounds being usually quite high.
Several methods can be used to incorporate salt fluxes in an alloy. U.S. Pat. No. 4,099,965 relates to a method where a flux of KCl and MgCl2 is added in solid form in the bottom of a preheated container before the addition of aluminum. More currently, fluxes are added by an inert gas in a pipe under the surface of the metal (lance fluxing). Recently, a method was developed where a hollow shaft brings the salt flux in the alloy with a gas carrier, and the salt flux is dispersed by an agitator (rotary flux injection). This method reduces the amount of salt flux required for carrying out the purification while increasing the dispersion of this salt flux in the alloy. Following the addition of a salt flux to the metal, impurities and salts float on the surface of the liquid metal and can be easily removed.
Advantageously, the use of solid compounds obtained by melting of salts controls the granulometry. Particles may be used in batch processes or in continuous processes.
However, costs related to salt fluxes such as binary mixtures of magnesium chloride and potassium chloride, are high. Furthermore, the use of salt fluxes having a substantial content in sodium chloride is not recommended by those skilled in the art due to perceived negative effects of sodium content in the resulting aluminum or aluminum alloys. In fact, when sodium chloride is present in fluxes for the purification of aluminum or aluminum alloys, those skilled in the art currently will avoid or limit the use of sodium chloride. More particularly, in the case of certain kinds of alloys such as, for example, aluminum alloys having silicon content higher than 10% by weight and more particularly aluminum alloys having magnesium content higher than 3% by weight, those skilled in the art currently recommend not using sodium chloride in salt flux.
During Applicant's search for a more effective solution to the purification problem, it was surprisingly noted that contrary to current apprehensions and beliefs of those skilled in the art, it is possible in a salt flux containing MgCl2, to replace expensive KCl by inexpensive NaCl. Consequently, the present invention offers an economical solution for the treatment of aluminum or aluminum alloys with an efficiency of purification that is equivalent to methods presently used. Indeed, contrary to apprehensions of those skilled in the art, there is no significant amount of sodium in the resulting aluminum or aluminum alloys when using the inventive purification method described herein.
Embodiments of the present invention show the following advantages:                Economical advantages                    Lower production costs because the melting point of the flux is lower.            Lower costs of raw material.                        Efficiency equivalent to the purification methods using an existing well known salt flux sold under the trademark Promag (40 wt % KCl-60 wt % MgCl2).        Economical alternative to existing product sold under the trademark Promag without creating any significant accumulation of sodium within aluminum or aluminum alloys.        